Geology Reference
In-Depth Information
A sediment or rock can be porous (having space between its particles) but
not permeable if the spaces are not connected in a way that allows flow of water
from one space to another. Similarly, sediment or rock can be permeable with low
porosity (not much space between the particles), such as clay.
When groundwater flows, it is responding to gravity, just like surface flow in streams.
Groundwater flow is also affected by pressure from the earth materials piled above it.
The combination of gravity and pressure is called the groundwater's potential. When
groundwater flows, it flows from areas of high potential (a high pull of gravity and high
amounts of pressure) toward areas of low potential. Because potential is calculated us-
ing the force of gravity, areas of low potential are commonly downhill from areas of high-
er potential.
The difference in groundwater potential across a measure of distance (like a
mile or kilometer) is called the hydraulic gradient, which indicates the direction of
groundwater flow.
Setting the water table
Scientists describe two zones of rock and sediment in the groundwater system. Water
moves first through a layer of sediments or soil called the zone of aeration. In this zone,
most of the space between sediment particles is filled with air, and water can move
through it. At some point, the water reaches a place where the space between sediment
particles is filled with water instead of air; this is called the zone of saturation. The zone
of saturation is what you aim for when you dig a well. Figure 12-7 is a sketch of the differ-
ent layers water moves through as it becomes groundwater beneath the surface.
Figure 12-7:
Beneath the sur-
face as water in-
filtrates sediment
and rock layers.
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